1. Field of the Invention
This invention relates to measuring and testing corrosion processes, and it relates particularly to the instruments and electrochemical techniques used in the study of corrosion processes.
2. Description of the Prior Art
It is often desirable to determine the rates at which metals corrode within a corrodant such as a corrosive liquid. For example, corrosion inhibitors are added to aqueous liquids to reduce the corrosion of exposed metals. Instruments are used to measure the rate at which these metals corrode so that the effectiveness of the inhibitor can be determined. The measure of the rate of corrosion upon metals usually involves an instrument associated with a probe carrying electrodes immersed within the corrodant. These instruments are usually termed "corrosion ratemeters". The electrodes in the corrodant undergo certain electrochemical changes that are related to the corrosion of the specimen forming the test electrode. The rate of corrosion can be correlated with the electrochemical effects upon the test (specimen) electrode contacted by the corrodant.
An electrochemical process and apparatus, especially useful in measuring corrosion rates, is described in U.S. Pat. No. 3,406,101. In this patent, there is described a corrosion ratemeter which includes a probe having three electrodes adapted to be exposed to a corrodant such as a corrosive liquid, an adjustable current source, an ammeter and a high impedance voltmeter as primary components. The adjustable current source applies a small electric current between a "test" electrode and an "auxiliary" or third electrode. At the same time, the voltmeter monitors the polarization potential produced by current flow between the test electrode and a reference electrode. The current flow slightly polarizes the surface of the test electrode, and as a result, causes a shift in potential (polarization potential) between the test and reference electrodes. The current flow required to produce a certain desired polarization potential (usually about 10 millivolts) is directly proportional to the corrosion rate of the test electrode undergoing corrosion. Usually, the polarization is selected within the linear voltage corrosion rate environmental conditions, and good results have been obtained with between 5 and 20 millivolts, but preferably 10 millivolts, polarization.
In corrosion ratemeters of the type described in the mentioned patent, the amount of current which flows between the test and third electrodes to produce a certain polarization potential is measured to determine the rate of corrosion occurring at the test electrode. Any potential differences, such as the freely corroding potential, present between the test and reference electrodes, other than the polarizing potential, produce erroneous results in corrosion rate determinations. Obviously, these instruments can produce highly accurate results when a correction is made for the non-polarizing potential differences between these electrodes, principally the freely corroding potential, which may exist between these electrodes before, and also during, application of the polarizing current. Additionally, in these instruments correction for the freely corroding and other potentials must be made without effecting the impedance between the reference and test electrodes immersed within the corrodant. Otherwise, a constant polarizing current will produce corresponding variations in polarization potential to be developed between these electrodes.
The corrosion ratemeter can employ suitable circuitry to provide automatically an equal but opposing potential for removing the freely corroding potential from the circuit including the test and reference electrodes. The freely corroding potential is usually less than 100 millivolts and frequently is about the same magnitude as the certain polarization potential (generally 10 millivolts) to be established by controlled current between these electrodes. Thus, the input potential correcting circuitry is required to sense and then automatically remove a potential of a very small magnitude from substantially the same magnitude of the certain polarization potential to be established between these electrodes.
An automatic correcting type of corrosion ratemeter using the controlled current mode of operation is of great advantage. Circuitry is required during "null" operation to compensate for the freely corroding potential which exists as the potential difference between the reference and test electrodes in a freely corroding state when no external current flows between the electrodes. During the "measurement" operation of such corrosion ratemeter, the circuitry must provide a reference signal which produces a current flow between the third and test electrodes. This current must be of such a magnitude that a certain polarization potential (5-25 mv) is produced between the test and reference electrodes in addition to the freely corroding potential. Additionally, a readout means must measure the output current creating this polarization potential which current magnitude is representative of the corrosion rate occurring at the test electrode.
Various circuits have been developed for automatic correcting types of corrosion ratemeters which can perform the above functions in the proper sequence and with the necessary accuracy of measurement. Reference may be taken to U.S. Pat. Nos. 3,661,751, 3,717,566 and 3,730,869 for examples of automatic correcting types of corrosion ratemeters available in the marketplace.
In some applications, these corrosion ratemeters are required to provide simultaneous readouts in several modes, e.g., voltage recorders, remote current transmitters, digital or analog signals and alarm circuits. Presently, interface modules are usually used to convert conjunctively the current flow between the third and test electrodes into output signals suitable with the desired output. Alternatively, the interface modules may use as their inputs voltage signals from recorder outputs. As a result, each interface module requires a separate signal input base and has individual calibration requirements.
The present invention is an automatic correcting type of corrosion ratemeter wherein the basic "null" and "measurement" circuitry can be of suitable design as in the above mentioned patents. As an improvement to these corrosion ratemeters, this invention provides a unique circuitry wherein all readouts have a common base for all signals. These signals have the same relationship to one another throughout the dynamic range of these corrosion ratemeters. As a result, calibration of these corrosion ratemeters for any one readout simultaneously produces the precise calibration of all other readouts. In addition, the readouts are steady state values during both null and measurement operations as contrasted with the more conventional asynchronous readouts that complicate both analog and digital data processing on varying readouts.